In the semiconductor industry, a bond pad refers to a contiguous metal pad typically formed out of the last layers of metal during a semiconductor manufacturing sequence. A bond pad is typically large enough to accommodate the bottom portion of a solder ball. A bond pad structure refers to a structure containing such a bond pad and surrounding or attached structure, which as a whole helps accommodate the solder ball.
Once the fabrication of integrated circuit elements on a semiconductor substrate is completed, the semiconductor substrate is diced and packaged in a bonding process. Bond pads provide a structure for electrical connection between the fabricated integrated circuit elements and the package. Typically, one end of an interconnection wire is bonded to a bond pad and the other end is bonded to the next level of integration, which is typically an inner lead of the package. In a typical bonding process, multiple interconnection wires are utilized to connect each of the electrically active pads to one of the inner leads of the package.
A typical bond pad structure contains an exposed large piece of metal on which a bonding wire is attached with a solder ball. During the operation of the chip, the temperature of the chip rises, thereby raising the temperature of the bonding structure including the bonding pad and the solder ball. Due to the differences in the coefficients of thermal expansion (CTE), the bonding structure is subjected to shear and stress. These may cause cracks in the bonding structure causing electrical failure of the bonding pad or slow degradation and reliability problems due to ingress of ambient atmosphere, especially moisture into the chip. Wakharkar et al., “Materials Technologies for Thermomechanical Management of Organic Packages,” Vol. 09, Issue 04, November 2005, pp. 309-324” discusses various aspects of reliability due to chip-package interaction (CPI).
Therefore, mechanical strength of the bonding structure that is sufficient to withstand the stress and shear during the operational lifetime of a chip is of utmost importance in the design of a bonding structure. To establish the reliability of a particular bond structure, it is customary in the semiconductor industry to subject the bond structures to rigorous stress routines and measure their failure rate. The standard method of testing the mechanical strength of a bonding structure is known as “JEDEC Standard” and is widely used in the semiconductor industry.
Many designs to enhance the mechanical strength of the bonding structure are known. As an example, U.S. Pat. No. 6,365,970 to Tsai et al.; U.S. Pat. No. 5,739,587 to Sato; and U.S. Pat. No. 5,700,735 to Shiue et al. utilize multiple layers of metals and via plugs. These structures utilize multiple layers of metals connected with via plugs in the bonding pad area to mechanically strengthen the bonding structures. One disadvantage of this approach is the lack of availability of the bonding pad area for wiring purposes. In other words, since multiple metal levels are filled with structures that are part of the bond structure, no other electrical structure such as metal wiring can be built within the same space. Thus, metal wiring is severely limited under the bond pad.
An alternative approach in the prior art that maximizes the available space for wiring under the bond pad is also known. Instead of utilizing multiple layers of metal, only the top level of metal is utilized for the bond pad. An electrical connection from the bond pad to lower metal levels is provided through an extension of the bond pad and vias attached to a lower level metal wire. The area below the bond pad is available for electrical wiring. If electrical wiring is not needed under the bond pad, the area in the lower level under the bond pads may be filled with metal fills to facilitate a chemical mechanical planarization (CMP) process.
It has been discovered during the process of the present invention that the above structure with one level of metal for the bond pads is prone to fracture when subjected to reliability stress. While the structure above provides maximum flexibility for wiring, the mechanical strength of the structure is not sufficient to provide a reliable structure under stress.
Therefore, the need exists to provide a bond pad structure that provides sufficient mechanical strength while still providing as much flexibility in metal wiring under the bond pad structure as possible.